Sound recording



Nov. 15, 1938. 5. F. MILLER 4 SOUND RECORDING Original Filed June 24, 1935 I 2 Sheets-Sheet 1 PIE. 3. 23

INVENTOR. Burn-0N F. M/LLEI? ATTORNEY.

Nov. 15, 1938. B, WLLER 2,136,724

' SOUND RECORDING I Original Filed June 24, 1935 2 Sheets-Sheet 2 INVENTOR. BUQTON i. M/LLEQ ATTO Y.

Patented Nov. 15, 1938 PATENT OFFICE SOUND RECORDING Burton F. Miller, Hollywood, Calif., assignor to United Research Corporation, Burbank, C'alif.,

a corporation of Delaware Original application June 24, 1935, Serial No. 28,109. Divided and this application April 4,

1936, Serial No. 72,730

2 Claims. (Cl. 179-1003) This invention relates to light valves for photographically recording sound upon a moving light a sensitive film.

On'e object of the invention is to record on a moving film a graphic representation of sound waves in such a manner as to have the graphic representation of as large an amplitude as possible with a relatively small movement of the speech current carrying conductor.

This is accomplished by providing an aperture through which a beam of light is projected upon a uniformly moving light sensitive film. The aperture is in the form of an isosceles triangle having a very small height in comparison to its 15: base. A speech current carrying conductor is placed over thetriangular aperture parallel to itsbaseso as to cover part of the aperture when no current is flowing. When a modulated alternating current ispassed through the conductor 20. the interaction between the magnetic force set up by the modulated currentand a strong magnetic field provided adjacent the aperture causes the conductor to vibrate in accordance with the amplitude and frequency of the sound being recorded. 25, This vibration allows a greater orless' length of the aperture to be exposed to the beam of light.

Another object of the invention is to keep the zero line or center of modulation of the wave forms always in the center of the sound track.

Another object of the invention is to reduce the ground noise due to the grain of the emulsion, or to dirt, oil, etc., adhering to the clear portions of the sound track (positive film) Heretofore; sound recording devices of the variable areaj type obtained ground noise reduction either by biasing the current carrying vibratory conductor or by the use of auxiliary shutters which were automatically controlled to follow the envelope of the wave forms and thus allow only 40, the areas between the successive wave forms to be exposed. Both of thesemethods have inherent disadvantages; the use of shutters entails rather complicated and sensitive apparatus which, due to electrical or mechanical lag or 'to faulty ad- 5, justment, may cause clipping of the sound peaks. Especially is thistrue in the case of stacatto type sounds where the rise in amplitude is practically instantaneous. When biasing of the conductor is employed, the zero or center of modulation is moved as close as possible to the edge of the clear portion of the sound track; that is, all the peaks of both the high and low level sounds are as close as possible to one edge of the track. Due to the shrinkage of the film or inaccuracies in the reproducing machine, the film thus biased has a tendency to weave sideways when passing the point of sound translation; this weave is at times sufiicient to allow one end of the light slit to cut off some of the wave peaks; although the peaks of the high level sounds may be cut off a substantial amount without any apparent distortion of the reproduced sound, this is not true of the low level peaks.

By keeping the center of modulation always in the center of the sound track, there will be no chance of clipping the low level peaks.

The present application constitutes a division of my co-pending application, Serial No. 28,109, filed June 24, 1935. The above co-pending ap plication discloses and claims the light valve per se which is shown and described herein.

Referring more particularly to the drawings,

Fig. 1 is an enlarged fragmentary view of the triangular aperture and its accompanying opaque speech current carrying conductor.

Fig. 2 is a sectional view taken along the line 22 of Fig. 3 showing my improved form of ligh valve for recording variable area.

Fig. 3 is a sectional view taken along-the line 3--3 of Fig. 2.

Fig. 4 is a schematic view showing the application of noise reduction to my improved form of light valve.

Fig. 5 is an enlarged fragmentary view of the section of sound film showing a double edged variable area sound track as recorded by my improved light valve.

Fig. 6 is an enlarged fragmentary view of a section of sound track on a negative film.

Fig. 7 is an enlarged fragmentary view of a section of sound track on a positive film.

Fig. 8 is a diagrammatic view showing the method of regulating the noise reduction.

Referring now to-Figs. 2 and 3, the light valve I comp-rises an upper magneticframe 2 having a pair of depending legs 3 and 4. A lower base plate 5 is secured'to the legs 3 and 4 as by screws 6. The frame 2 has a hollow core 1 comprising one of the pole pieces and is provided with a magnetizing coil 8. Direct current of as constant a value as possible is passed through the coil 8 to produce the high magnetic flux between the core 1 and a pole piece 9 secured to the base plate 5 in substantial alignment with the core I. The pole piece 9 and also the base plate 5 are provided with an aperture ID to allow a beam of light to be projected therethrough. The triangular aperture l I comprising the light slit is formed by means of three plates I2, l3, and I4, (Figs. 1 and 2) which are secured to the upper surface of the pole piece 9 by means of a pair of clamping pieces I5 and I6. Clamping screws I1 serve to clamp" the pieces I5 and H5 in position. The aperture I I is of very small height in comparison to its base, being, in this instance, about 1 mils high and about 140 mils long. A pair of bridges I8 and I9 is provided, one on either side of the pole piece 9 parallel to the base of the aperture II. These bridges I8 and I9 are preferably composed of an insulating material such as Bakelite or hard rubber and are secured to the base plate 5 as by screws 29 and 2|. The forward edges of the bridges I8 and I9 have plates 22 and 23 secured thereto as by screws 24 and 25. These plates 22 and 23 project above thesurface of the bridges I8 and I9 and serve as supports on which a vibratory conductor 21. is placed. Clamping pieces 28 and 29 are secured to the plates 22 and 23 as by screws 39 and 3| (Fig. 2). Each of the bridges I8 and I9 is provided with a pair of adjusting jaws 32 and 33. These jaws are p-referably of a spring material and are secured to the rear ends of the bridges as by screws 34 and 35. The forward ends of the jaws 32 and 33 are engaged by adjusting screws 36 and 31. Adjustment of the conductor 21 is accomplished by rotating either of the adjusting screws'36 and 31. When the proper adjustment has been effected, the conductor 21 is clamped in position by means of the clamping pieces 28 and 29. Theends of the conductor 21 are secured to windlasses 39 and 39 which are rotatably secured within insulated blocks 40 and 4|. Beforereaching the Windlass 39, the right hand end of the conductor is looped over a spring pressed arm BI which is pivotally mounted to the base as at 62. The arm SI allows for easier adjustment of the tension of the ribbon and prevents undue'tensionfrom being applied. The blocks 49 and M are secured'to the base plate 5 by means of screws 42. Each of the windlasses 38 and 39 is electrically connected to terminals 43 and 44 (insulated from the'base plate 5) which in turn are adapted to be connectedto a suitable sound amplifying system.

Referring to Fig. 4, I will now describe the application of noise reduction to the above mentioned type of valve. The sounds to be recorded upon a light sensitive film 45 are received in a microphone 46 and amplified by a suitable amplifier 41. The speech currents thus amplified are passed through a transformer 49 and thence by means of conductors 49 and 59 are passed through the vibrating conductor 21. Interaction between the magnetic force set up by the modulated alternating current passing through the conductor 21 and the strong magnetic field set up between the pole pieces 1' and 9 causes the conductor 21 to vibrate in accordance with the amplitude and frequency of the sound waves and thus vary the length of the triangular aperture II (Fig. 3) exposed to a beam of light from an exciter lamp 52. Light from the exciter lamp 52 is collected by means of a condenser lens 53 and is focused at a point adjacent the slit II from whence it is projected onto the moving film 45 by means of an objective lens 54. Noise reduction is obtained by holding'the center of modulation of the conductor 21 in a position as. shown by the dot and dash lines 55 (Fig. 1) when no or little sound is being recorded. This is accomplished by passing a biasing current from a battery 6| (Fig. 4) through the conductor 21. An amplifier 56 and rectifier 51 are shunted across the conductors 49 and 50. In recording low level sounds, a very small amount of current is passed through the amplifier and rectifier combination 56 and 51 but when high level sounds are recorded, a correspondingly greater amount of current is passed through the amplifier 56 and rectifier 51. The resulting rectified current is passed through the conductor 21 in a direction opposing the steady biasing current from the battery 6I thus neutralizing the biasing effect, Thus the center of modulations for high level sounds is moved to a position shown by the full lines 21 (Fig. 1).

The biasing of the conductor 21 may also be obtained by applying the rectified current from the rectifier 51, which varies in accordance with the amplitude of the sounds being recorded, directly to the conductor 21. In this case, the normal position of the conductor at low level recording would be mechanically adjusted so that it would cover a greater part of the slit II as indicated by the dot and dash lines 55 (Fig. 1). With the increase in amplitude of sound recorded, the corresponding increase of the rectified current from the rectifier 51 would shift the center of modulation of the conductor 21 toward the position shown by the full lines (Fig. 1).

The amount of biasing of the conductor is generally dependent upon the kind of sound being recorded. When recording dialogue or music in which the volume is low or where the change in volume is gradual the mean biasing position of the conductor may be brought nearer the apex of the triangular aperture than in the case of recording staccato sounds such as are produced by a harp or piano. In recording the latter type of sound, more leeway has to be given to the conductor to prevent the higher level wave forms from being clipped before the conductor has a chance to be moved toward its unbiased position. This change in biasing position of the conductor is accomplished by means of suitable potentiometers as at 58 and 59 (Fig. '1) and may be effected during the recording wherever the tempo of the sound is changed. The amount of biasing is generally determined by the amount of margin desired between the opposite depressions of a wave form, for example, D1 and D2 (Fig. '1) Referring to Fig. 8, the margin may also be determined by the distance from the apex of the triangular aperture to the position of conductor 21 at the upper limit of its excursions as shown by the dotted lines 60. In setting the biasing position of the conductor 21, the potentiometers 58 and/or 59 are manipulated until the distance X, representing one-half the amplitude of the conductor excursions, is allowed to reach a certain percentage of the distance D; this, of course, depending on the type of sound being recorded. In recording soft sounds, the distance X may be allowed to cover as much as 75% of the distance D.

In the example shown in Fig. 8, the potentiometers 58 and/or 59 is set so that X will equal onehalf D when the conductor is in its biased position. Thus the margin for low level sounds is indicated by the dimension M. When X exceeds one-half of D the change in biasing current causes the mean position of the conductor to be moved downward so as to maintain the same ratio of X to D. This ratio is kept constant until the conductor 21 reaches the unbiased position as shown by the dotted lines 21' at which point the mean position of the conductor 21 will lie half way between the apex and the base of the triangle. Here the margin M will be seen to be considerably larger allowing for an increase in ground noise. This is, however, negligible due to the fact that the sound level is higher in proportion. As higher amplitudes are recorded, the center of modulation of the conductor 2'! remains the same while the margin again becomes smaller as at Mh to accommodate these higher amplitudes.

Referring to Fig. 5, the center of modulation for each edge of the positive sound track is indicated by the center lines BI and 62. Where no sound isrecorded on the film, only a narrow path of transparency 63 traverses the center of the sound track. As modulations are recorded and increased, the centers of modulation 6! and 62 corresponding to the mean biasedposition of the conductor 2'! are spread apart until they reach their unbiased position. I

Due to the fact that the edges of the sound tack are formed by the acute angle between the made extremely sensitive to the shading of the negative so that except for a small zone 68, the clear portion 69 will be entirely transparent. The

amount of distortion produced by the shaded zone 63 is negligible due to the fact that its dimension as measured transverse of the direction of film travel remains constant and thus is unable in itself to effect a varying change in light passing therethrough; it being understood, of course, that the reproduction of sound isdependent upon a change in the amount of light emanating from a slit placed transverse of the film travel.

Having thus described the invention .what is claimed as new and desired to secure by Letters Patent is:

1. Means for recording an anti-ground noise type of variable area sound record comprising a single electrical conductor, means forming a triangle aperture and cooperating with said, conductor, means for directing a beam of light through the opening between said aperture and said conductor, means for vibrating said conductor across said aperture in accordance with sound modulated current and means for varying said opening in accordance with the average volume of said current.

2. An apparatus for recording an anti-ground noise type of sound record comprising means forming a triangular aperture, a single' vibratile electrical conductor, vibratile at the frequencies to be recorded, said conductor being positioned adjacent said aperture forming means and adapted to vibrate across said aperture, means for directing a beam of light through the opening between said aperture and said conductor, means for simultaneously vibrating'said conductor in accordance with both sound modulated current and current varying in accordance with the average amplitude of said modulated current, and means for moving a light sensitive film across said light beam modulated by said conductor.

BURTON F. MILLER. 

